I've been trying to activate my iPhone 4 but it says "Your iPhone could not be activated because the activation server cannot be reached" I always try and changed the WiFi but wont work!

If you can't activate your iPhone - Apple Support

@ QuickTimeKirk: Diego wrote that the activation server cannot be reached, he never wrote that he was asked for an Aplle-ID!

@Diego: Just wait and try again. I had it more then once that the Apple server wasn't reachable.

How to activate

So how

That sounds very reasonable for such an old device.

If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support

Is this a used device?

Take it back to where you bought it and ask for your money back.

It's got the previous owner locked to it and you'll never activate it.